The water boils a lot, either a cup of tea prepared in the kitchen or a power plant that generates electricity. Any improvement in the efficiency of this process will have a major impact on the total amount of energy used each day.
One of these improvements could come with a newly developed treatment for surfaces involved in water heating and evaporation. The treatment improves two key parameters that determine the boiling process: the heat transfer coefficient (HTC) and the critical heat flow (CHF).
Most of the time, there is a compromise between the two: as one gets better, the other gets worse. After years of research, the research term behind the technique has found a way to improve both.
“Both parameters are important, but improving the two parameters together is a bit tricky because they have an intrinsic commitment,” says bioinformatics scientist Youngsup Song of the Lawrence Berkeley National Laboratory in California.
“If we have a lot of bubbles on the boiling surface, that means the boiling is very efficient, but if we have too many bubbles on the surface, they can stick together, which can form a vapor film on the boiling surface. ‘boiling’.
Any vapor film between the hot surface and water introduces resistance, reducing heat transfer efficiency and CHF value. To solve the problem, the researchers devised three different types of surface modification.
First, a series of microscale tubes are added. This series of tubes 10 micrometers wide, separated by about 2 millimeters, controls the formation of bubbles and keeps the bubbles fixed to the cavities. This prevents a vapor film from forming.
At the same time, it reduces the concentration of bubbles on the surface, reducing the boiling efficiency. To do this, the researchers introduced an even smaller-scale treatment as a second modification, adding bumps and ridges of only nanometers in size to the surface of the hollow tubes. This increases the available surface area and favors evaporation rates.
Finally, the microscale cavities were housed in the center of a series of pillars on the surface of the material. These pillars speed up the liquid extraction process by adding more surface area. In combination, the boiling efficiency increases significantly.
(Song et al.)
Above: A slow-motion video of the researchers ’setup shows boiling water on a specially treated surface that causes bubbles to form at specific separate points.
Because nanostructures also promote evaporation under bubbles and the pillars maintain a constant supply of liquid to this bubble base, a layer of water can be maintained between the boiling surface and the bubbles, improving heat flow. maximum.
“Showing that we can control the surface this way to improve it is a first step,” says mechanical engineer Evelyn Wang of the Massachusetts Institute of Technology. “So the next step is to think of more scalable approaches.”
“These kinds of structures we’re doing aren’t meant to be scaled in their current form.”
Bringing the work of a small-scale lab into something that can be used in commercial industries won’t be that simple, but researchers are confident it can be done.
A challenge will be to find ways to create the surface textures and the three “levels” of modifications. The good news is that there are different approaches that can be explored, and the procedure should also work for different types of liquids.
“That kind of detail can be changed, and that can be our next step,” Song says.
The research has been published in Advanced Materials.